At the present time there exist various devices for vehicle location, vehicle identification and cargo identification. These devices have required a large variety of hardware to obtain automatic identification or location of the vehicle or cargo. In some devices or systems optical techniques have been employed. However, these are susceptible to errors caused by misorientation and foreign matter on the interrogator or label.
A large number of RF systems have been devised over the years. However, these too have many serious shortcomings. Many rely upon reflecting the fundamental frequency or altering its polarization or phase. These techniques suffer from practical problems involving sensitivity of the equipment and the likelihook of false alarms. In addition, if multiple frequency codes are used, large amounts of spectrum may be required to convey a complex digital identification number. One alternative to this is some form of mechanical scanning which several of the previous systems or devices have utilized. However, these techniques require the same alignment restrictions as the previous optical techniques and are therefore unsatisfactory for many road vehicle and cargo applications. Some others of these systems or devices use serially encoded information; however, they suffer from problems of practicality. For instance, some such systems or devices use either a core memory or a magnetic disc which is motor-driven to store the identification information; however, the power levels present in a radiation-powered transponder are usually insufficient to operate these systems or devices in a satisfactory manner. Finally, there is a class of RF devices which uses one or more solid-state oscillators which are modulated by coded information and which are powered by incident RF energy or from a battery. While these systems or devices do not have some of the previous shortcomings, their transmitters require more DC power and they are complicated and costly.
The present invention overcomes all these foregoing disadvantages and provides a simple, inexpensive, small, passive transponder which may be readily mounted in many orientations on the object to be identified and permits unambiguous identification and reception of the transponder signal and a method of using such a transponder.